Non-linear video signal processing

Abstract: A video input signal (IN) is applied to a bridged-T attenuator (R1,R2,R3,20) which includes a non-linear element (20) in the shunt branch thereof that provides increasing attenuation as the input signal increases to thereby produce a gamma corrected video output signal at the attenuator output. A current sensor in the shunt branch generates an exponentially increasing detail signal (S3) for increases in the video input signal. The detail signal is high pass filtered (24) and combined with the gamma corrected signal (S7) by a summing circuit (18) for providing a video output signal (OUT) having gamma correction and high frequency detail augmentation.

Description: This invention relates to television systems generally and video signals. In an ideal television system the light output produced by a kinescope would be linearly related to the light applied to a camera pick-up tube. However, in practical systems, the camera tube and the display tube are both non- linear devices, and the overall camera/display net "gamma" (which represents the curvature of the transfer function) differs from a value of unity which would be necessary for linear response. The non-linearities, if uncorrected, would tend to reduce the contrast and to make detail in dark picture areas almost invisible while the whites would tend to be amplified to the point of picture tube saturation and blooming. Correction of non-linearities (i.e., gamma correction) requires non-linear processing of the video signal to be displayed. An example of conventional gamma correction apparatus is described by Haferl in European Patent Application 0 427 564 A3 entitled VIDEO SIGNAL PROCESSING which was published 15 May 1991. In the Haferl apparatus a video signal to be gamma corrected is applied directly to a summing circuit via a linear signal path (e.g., a resistor). The video signal is also applied to a non-linear signal splitter which generates a first gamma correction signal for black to gray picture areas and a second gamma correction signal for gray to white picture areas. The first and second gamma correction signals are coupled via respective ones of a DC signal coupling path and an AC signal coupling path to the summing circuit. This system, however, is relatively complicated in requiring splitting of the gamma processing into level dependent ranges. Another example of gamma correction apparatus is described by Monahan in US patent No. 4,057,828 entitled CONTRAST COMPRESSION CIRCUITS which issued 8 November 1977. In the Monahan apparatus wide band video signal to be compressed is first cored to remove noise. A compression difference signal is then developed by processing the wide band video signal with a conventional gamma circuit and subtracting the wide band video signal from the processed signal. An analog multiplication circuit receives the difference signal and multiplies it by a picture gamma control signal. The controlled difference signal is then added back to the uncored wide band signal to provide the amplitude compressed wide band signal. Compression is controlled by varying the factor by which the difference signal is multiplied. This system, however, is relatively complicated in requiring analog multiplication. It is an object of the present invention to provide gamma correction of simplified form without requiring range splitting or relatively complex multiplication operations. The principles of the invention have application to gamma correction apparatus of a type comprising a source (12) for providing a video signal to be gamma corrected; a summing means (16) for providing a gamma-corrected video signal; a first signal path (R3) for providing coupling between the source (12) and the summing means (16); and a second signal path (R1, R2) coupled between the source (12) of video signal and the summing means (16) for developing and coupling a gamma correction signal to the summing means (16). Gamma correction apparatus embodying the invention is characterized by non-linear function means (20) coupled between the second signal path and a source of reference potential (ground) for by-passing a portion of the video signal to the source of electrical potential in response to a function of the level of said video signal. In a preferred application of the principles of the invention, the second signal path includes a circuit node (14) coupled via a resistor (R1) to the video signal source and the non-linear function means (20) comprises: a transistor (Q1) coupled between the circuit node and the source of electrical potential; and a feedback control circuit (R4, R5) is coupled between the circuit node (14) and a control input (base) of the transistor for varying the conduction thereof in proportion to the level of the video signal at the circuit node. In accordance with a further feature of the invention, in a preferred application of the principles thereof, the feedback control circuit comprises: a potential divider (R4, R5) coupled between the circuit node (14) and the source of reference potential and having an output coupled to apply a given fraction of a potential developed at the circuit node to the control input (base) of the variable conduction device. In accordance with another feature of the invention, for providing frequency independence of the potential divider, a first capacitor (C1) is coupled in parallel with the resistor (R1) in the first signal path and a second capacitor (C5) is coupled between the output of the potential divider and the source of reference potential. The foregoing and further features of the invention are illustrated in the accompanying drawing, wherein: FIGURE 1 is a simplified block diagram of a non-linear video signal processor embodying the invention; FIGURE 2 is a detailed circuit diagram with exemplary component values of the processor of FIGURE 1; FIGURES 3-5 are transfer function diagrams illustrating low frequency response characteristics of the processors of FIGURES 1 or 2; and FIGURES 6-9 are transfer function diagrams illustrating high frequency response characteristics of the processors of FIGURES 1 or 2. As an overview, the non-linear processor of FIGURE 1 includes a non-linear function network 20 in the shunt branch of a bridged-T attenuator. Advantageously, the non-linear function network 20 provides the dual functions of: (1) increasing the bridged-T attenuation as a function of the amplitude of the video input signal so as to provide gamma correction to the video output signal; and (2) providing a sensed video signal that is exponentially related to the amplitude of the video input signal for use in enhancing the high frequency detail of the gamma corrected video output signal. In more detail, the non-linear processor of FIGURE 1 includes an input terminal 10 for receiving a video input signal S1 and an output terminal 28 for providing a processed video output signal S2 having gamma correction and high frequency detail enhancement. The bridged-T attenuator includes an input node 12 that is coupled to input terminal 10 and, via a first resistor R1 to a central node 14 which is coupled via a second resistor R2 to the output node 16. The bridging resistor R3, coupled between nodes 12 and 16, determines the maximum attenuation of the attenuator and the influence of the non-linear function network attached to the central node 14. More specifically, bridging resistor R3 actually limits the amount of non-linearity because R3 is a linear by-pass. If resistor R3 is equal to the sum of the T resistors R1 and R2, then the maximum attenuation reached will be 6 dB. Increasing the value of the bridging resistor R3 will increase the maximum value of the attenuation provided. Further increases in the value of the bridging resistor will result in further increases in the maximum value of the attenuation and, in the limiting case, the bridging resistor R3 may be removed to obtain the highest level of video signal attenuation. Accordingly, the T-resistors R1 and R2, in relation to the bridging resistor R3, determine the attenuation range. The value of the attenuation is controlled as a function of the video signal amplitude by means of the non-linear function network 20 coupled between the central node 14 and a point of reference potential (illustrated here as ground). As the value of the video input signal increases, the impedance of network 20 decreases thus shunting more and more of the input signal to ground. Accordingly, for relatively small input signals the attenuation is also small and the output signal is substantially equal in magnitude to the input signal. Thus, signals near black level suffer little loss. However, as the input signal increases towards white level the attenuation also increases. Accordingly, the whiter the signal gets the more the attenuation it receives and so the transfer function exhibits a negative slope thereby applying gamma correction to the processed video output signal. As an illustration of the operation discussed above, FIGURE 3 illustrates a ramp video input signal 300 making a black to white transition. In FIGURE 4 the waveform 400 illustrates the voltage at the central node 14 caused by the attenuation (impedance) change of the non-linear function network 20. As seen, the voltage initially follows the input signal (FIG. 3) in the black to gray region but as the input signal goes from gray to white the rate of change of the node 14 voltage decreases. This results because at the higher levels the impedance of the network is decreasing and so more and more of the input current is shunted to ground. FIGURE 5 illustrates the waveform 500 of the shunt current S4 flowing through the non-linear network 20. It will be noted that this current increases as an exponential function of the input signal. By sensing the shunt current S4 in network 20 (e.g., by a resistor) a video signal S3 is produced that is suitable (as a starting point) for enhancing the high frequency detail of the processed output signal. This is done, as shown in FIGURE 1, by high pass filtering the signal S3 and applying the filtered signal S5 along with the gamma corrected signal S7 to a combiner (e.g., an adder) 18 for producing the processed video output signal S2. To avoid adverse noise effects in the high frequency detail signal S5 one may, if desired, apply the signal to a coring circuit 26 to achieve noise reduction. Some of the more salient features of using the non-linear network 20 for the additional function of high frequency detail augmentation are illustrated in FIGURES 6-9. FIGURE 6 corresponds to FIGURE 3 but illustrates the black to white ramp input signal 600 as also including high frequency (HF) details. FIGURE 7 illustrates the sensed voltage S3 proportional to the current though the shunt branch of the attenuator. As shown, the increase in detail exhibits an exponential relationship with the video input signal with the detail being enhanced at higher input signal levels. Waveform 800 of FIGURE 8 illustrates the effect of detail on the output signal with only the output signal S7 of the bridge used and with the augmentation signal S3 disabled. As seen, the processed signal clearly is gamma corrected but the detail shows little effect. When the detail signal S3 is enabled the effect, as shown by waveform 900 in FIGURE 9, is a substantial increase in detail in the gray to white video signal levels. FIGURE 2 provides an exemplary implementation of the video signal processor of FIGURE 1. The example of FIGURE 2 also includes additional advantageous features for enhancing the performance and reducing the number of circuit elements for implementing the non-linear video signal processor. These include: (1) frequency compensation of the bridged-T shunt branch; (2) manual gain "T" branch gain control; and (3) elimination of the need for a separate combiner circuit for the gamma corrected and detail signals. In more detail, in FIGURE 2 the bridged-T attenuator comprises resistors R1, R2 and R3 connected as previously described. The non-linear network 20, illustrated in phantom, comprises a PNP transistor Q1 having a conduction path coupled at the emitter end to the central node 14 via a diode D1 and coupled at the collector end to ground via a current sensing resistor R6. Bias for application to the control electrode (base) of transistor Q1 is provided by the attenuated video signal appearing at the central node 14 by means of a potential divider comprising resistors R4 and R5 coupled between node 14 and a reference potential point (ground). In operation of the non-linear network, the transistor Q1 in combination with diode D1 and resistors R4 and R5, acts as a current sink when the input signal level causes Q1 to conduct. With a linear increase of the input voltage (FIGURE 3), the current through diode D1 and transistor Q1 increases exponentially (FIGURE 5) while the current through resistor R2 decreases by the same amount, resulting in a smooth gamma correction (FIGURE 4). The voltage divider comprising resistors R4 and R5 determines the emitter current and operating point of transistor Q1. Diode D1 is not strictly necessary. However the presence of diode D1 advantageously doubles the voltage across resistor R4 for a particular emitter current and provides a more convenient ratio for the values of resistors R4 and R5. Together with the base-emitter capacitance of transistor Q1, which is quite high in the cut-off region, capacitors C1 (parallel with resistor R1) and C5 (parallel with resistor R5) provide the advantage of making the voltage divider frequency independent. When transistor Q1 is not in saturation, the emitter current rises exponentially with a linearly increasing input voltage. Thus, the voltage across resistor R6 varies in the same manner (FIGURE 5) This signal is buffered by transistor Q2 which with resistor R7 serves as an emitter follower. The buffered signal is coupled to the input of a high pass filter. The filter comprises resistor R8 and capacitor C8 and has a cut-off frequency, illustratively, of about 8 MHz providing a linear phase behavior at lower frequencies (e.g., in the video frequency range). As noted above, in this example of the invention no separate adder is required for summing the gamma corrected and high pass filtered detail signals. Instead, this function is performed by a kinescope driver amplifier 200 having a non-inverting input (+) coupled to a source of reference potential (Vref.), having an inverting input terminal (-) coupled via a feedback resistor RF to an output terminal 28 and coupled also to a point of reference potential (ground) via a resistor R9. Since the amplifier 202 has negative feedback provided by feedback resistor RF, the inverting input (-) acts as a current summing node for all the currents provided by resistors R2 and R3 of the bridged-T attenuator and the current passed by the high pass filter (R8, C8). In effect, the kinescope driver amplifier, in this example of the invention, serves the dual functions of both amplifying the video signal for application to a kinescope gun and for summing the three currents of the non-linear video 'signal processor. As noted above, a further feature of this embodiment of the invention concerns the provision for manual gain control. This is provided for in FIGURE 2 by use of a manually variable resistor (potentiometer) for resistor R2 in the T-attenuator portion of the bridged-T attenuator. This control is useful for providing a balance adjustment for R, G, B drive signals in applications where a separate non-linear processor is provided for each gun of a color television receiver. In applying the principles of the invention in a specific application, it is preferred that the non-linear processing be applied to color component signals such as, for example, red, blue and green. However, in a given application one may wish, instead, to apply the non-linear processing to a luminance signal. It should be noted, however, that gamma correction in a luminance signal path may lead to desaturation of colors.

Claims: Gamma correction apparatus, comprising: a source (12) for providing a video input signal to be gamma corrected; summing means (16) for providing a gamma-corrected video output signal; a first signal path (R3) for providing coupling between said source (12) and summing means (16); a second signal path (R1,R2) coupled between said source (12) of video signal and said summing means (16) for developing and coupling a gamma correction signal to said summing means (16); characterized by : non-linear function means (20) coupled between said second signal path and a source of reference potential (ground) for by-passing a portion of said video signal to said source of reference potential in response to a function of the level of said video signal. Apparatus as recited in Claim 1, wherein said second signal path includes a circuit node (14) coupled via a resistor (R1) to said video signal source and further characterized in that said non-linear function means (20) comprises: a transistor (Q1) coupled between said circuit node and said source of reference potential; and a feedback control circuit (R4,R5) coupled between said circuit node (14) and a control input (base) of said transistor for varying the conduction thereof in proportion to said level of said video signal at said circuit node. Apparatus as recited in Claim 2 further characterized in that said feedback control circuit comprises: a potential divider (R4,R5) coupled between said circuit node (14) and said source of reference potential and having an output coupled to apply a given fraction of a potential developed at said circuit node to said control input (base) of said transistor. Apparatus as recited in Claim 3 further characterized in that for providing frequency independence of said potential divider, a first capacitor (C1) is coupled in parallel with said resistor (R1) in said first signal path and a second capacitor (C5) is coupled between said output of said potential divider and said source of reference potential.

Inventors: KELLER ANTON WERNER, KELLER, ANTON WERNER
Applicants: RCA THOMSON LICENSING CORP, RCA THOMSON LICENSING CORPORATION
Application year: 1994
Main classification: H04N5
IPC codes: H04N5, H04N9